The present invention relates to a process for molding plastic containers and to the article produced by the process. Specifically, it relates to a unitary, injection molded plastic basket produced in a two-piece mold and having light weight, strength, and large carrying capacity.
Generally, the injection molding of a plastic article requires a mold formed of a number of pieces, which when fitted together define a cavity of the shape of the article. The mold is assembled and the cavity filled by forcing liquid thermoplastic material into it. After the plastic has cooled and set, the mold may be disassembled and the plastic article removed, at which time it may require further finishing.
One type of injection molding apparatus is exemplified by Fisher U.S. Pat. No. 3,152,365. This type of mold, called a sliding mold, has several dies which move into and out of engagement with each other in several directions and, possibly, at different times. In Fisher, for example, the injection molding apparatus is designed to produce the handle of a telephone handset. As shown in FIG. 1 of Fisher, in addition to a lower molding die and an upper molding die which move vertically into and out of engagement with each other, the mold includes two core members, forming the transmitter and receiver housings, which are rotated in and out of position along screw threads. The mold also includes two other core members, forming the interior surfaces of the central tubular portion, which must be slid in and out of position around the corners of the transmitter and receiver housings. Because such a mold has multiple members, it is capable of producing a plastic article of relatively complex shape. However, it has the disadvantages that a good deal of time is necessary to move the multiple members into and out of molding position and that the apparatus itself, because it has many moving parts, is relatively expensive. The sliding mold, therefore, is unsuitable for fabricating simple articles such as shopping baskets which are intended to be sold at low cost.
Another type of injection molding apparatus, exemplified by Long U.S. Pat. No. 2,556,590, is the two-piece mold. As its name implies, the mold of this apparatus consists of only two pieces: a core member and a cavity member. Furthermore, in the two-piece mold the core and cavity members are moved into and out of engagement in a single direction. The interior surface of the cavity, and the exterior of the core, when the two dies are placed together, define the shape of the plastic article. Because the two-piece mold has no moving parts, it is inexpensive both in initial cost and in operation and is therefore capable of producing an inexpensive plastic article. Its chief disadvantage arises from the need to withdraw the core die from the cavity die along a single direction. Because of this feature, plastic articles produced in a two-piece mold generally may not have vertical walls, that is, walls which extend in the direction of separation of the two dies. When vertical walls are attempted, they are torn from the rest of the article as the dies are separated. Typically, therefore, to facilitate separation of dies, the walls of the plastic article are designed with a considerable amount of draft, or taper. In fact, it is generally considered that such walls may be no closer than six degrees to the vertical without assuming substantial risk of destruction of the plastic article when the mold is separated.
There is also a relationship between the thickness of the wall and the amount of draft necessary to ensure its integrity upon mold separation. Generally, thinner walls require a greater draft. The reason for this is apparent. The thinner a wall, the lower the maximum shear force it can sustain without breaking. Ideally, for a wall of infinitesimal draft, when mold separation occurs the dies instantaneously separate from the molded article and no shear forces are applied to the article. Practically, however, there are attractive forces between the surface of the dies and the surface of the article, forces which decrease with the distance between the surfaces. The shallower the draft of the wall, the closer the surfaces are for a given distance of mold separation and the greater the force on the article tending to part it. Such shallow drafts, therefore, have previously required thicker walls to withstand the forces of mold separation. A problem develops, therefore, in designing a lighter weight, lower cost basket by thinning the walls. If the only weight-reduction measure taken is to reduce the thickness of the basket walls, their draft must be simultaneously increased. For a given maximum size of basket, thinner walls and a greater draft necessarily result in a smaller carrying capacity.
To overcome this problem of the relationship between wall thickness, draft, and weight, solid, thin walls have been replaced by thicker, slotted walls, as shown in the previously-mentioned Long patent. Molding the basket with slots in its walls has the added advantages that the contents are more visible and that they are ventilated. On the other hand, slots present their own set of problems, especially in molding a basket using a two-piece mold. Not only is the draft of the basket walls important, but the draft of the edges of the slots likewise must be controlled to avoid destruction of the article as the mold is separated. The same reasons apply here as were discussed above in connection with wall thickness. Because of the attractive forces between the dies and the edges of the slots, the slot edges must have sufficient draft to withstand mold separation.